Prior art ICD circuitry delivers energy to the patient by discharging a charged capacitor into electrodes that are in direct contact with the patient's heart. In those ICDs, total energy is limited by controlling the amount of stored energy on the capacitor, which, in turn, limits the charge voltage. Typically, the shock is truncated when voltage on the capacitor decays to a known value. The resulting waveform exhibits an exponential decay with a constant tilt. The tilt is the percentage by which the waveform voltage decays from start to end. Accordingly, patients with larger load impedances will receive longer shocks because it will take a longer time for the waveform voltage to decay.
Waveforms used in implantable defibrillators to defibrillate a patient have been, for the most part, truncated exponential waveforms. Truncated exponential waveforms are generated by charging the capacitor(s) and discharging it through the total impedance, which includes the impedance of the patient and the leads used to deliver the waveform. Truncation has been used clinically because of the concern that the long, low tail end of a non-truncated exponential waveform might re-induce fibrillation.
In spite of its limitations, manufacturers of implantable cardioverter-defibrillators (ICDs) continue to use truncated exponential waveforms for clinical settings. Moreover, ICD manufacturers have long produced ICDs with programmable shock strengths. The strength of the shock required to defibrillate, is controlled by the time constant (TC) and tilt (T) of the truncated exponential waveform. TC is defined as the time required for the shock voltage to decrease to a preset percentage of its starting value and T is the percentage of leading edge voltage remaining at the trailing edge of the waveform. Altering the duration of the waveform while maintaining the same TC will change T. Altering TC while holding waveform duration constant will change T. T can additionally be changed by modifying both TC and waveform duration.
Monophasic truncated exponential waveforms were generally used until biphasic truncated exponential waveforms were introduced. Biphasic waveforms are created by a switch in the capacitance that reverses the polarities delivered to the electrodes during delivery of the shock pulse. Some biphasic waveforms are thought to have lower defibrillation thresholds (DTs), compared to monophasic waveforms. This is particularly true when the first phase of the biphasic waveform delivers more energy than the second phase.
In addition to the types of waveform used, the determination of which electrode functions as the anode in the right ventricle appears to lower the DT when a monophasic waveform is used. Such a determination of the electrode polarity, however, appears to have little influence when biphasic waveforms are used. Clinicians, however, generally err on the side of caution and program the right ventricular electrode as the anode when using a biphasic waveform.
A published study by Huang et al., “Defibrillation Waveforms” in Nonpharmacological Therapy of Arrhythmias for the 21st Century: The State of the Art, Futura, 1998 concludes: “Thus, the (truncated exponential) biphasic waveform appears to be more efficacious for defibrillation than the (truncated exponential) monophasic waveform for internal as well as external defibrillation and for ventricular as well as atrial defibrillation.” This same study, in its opening paragraph, states: “Schuder, et al (in Circ Res, 1966, 19: 689–694) have shown that for external defibrillation in the dog, a waveform consisting of an ascending ramp has a much higher success rate for defibrillation than a descending ramp waveform of the same strength.” Despite this fact, there has been little research and/or implementation of the ascending ramp. This may be because waveforms similar to the descending ramp are much easier to generate, the descending ramp type of waveform is used clinically even though it is much less efficient for defibrillation.
In U.S. Pat. No. 5,725,560, Brink describes a method of delivering arbitrary waveforms with a computer-controlled system. The basic energy converter topology disclosed is a buck, or step-down, type of power converter with a pulse width modulation control scheme. This type of power converter is a common topology used in the field of energy conversion. The circuitry developed in the '560 patent is implemented as a full bridge (H-bridge) dc-dc converter that enables biphasic waveforms. The system monitors the voltage and current delivered to the patient and uses these parameters as a control feedback.
Weiss, in U.S. Pat. No. 5,184,616, teaches an arbitrary waveform circuit for use in ICDs. As in the '560 patent, a switching power converter is used with a full bridge (H-bridge) implementation. The '616 patent has a control scheme with a predetermined pulse width or duty cycle for each switching cycle during delivery of the waveform. In some cases, an impedance measurement is required to determine the proper timing. This impedance measurement uses a constant current source by applying current to the patient and then computing measured applied voltage over applied current. A feedback element receives signal information from the output of the filter circuit. Based on this input, the circuit assumes that the output to the patient is monitored so that the microprocessor can make adjustments to the shock control, charge control, and dump control lines.
Imran, in U.S. Pat. No. 4,768,512, describes a method of delivering a truncated exponential waveform that is “chopped” or comprised of a train of high frequency, exponentially decaying pulses delivered from a storage capacitor. In this patent, when a feedback signal on the patient load drops below a reference voltage, the output voltage is disabled, resulting in a waveform truncation.
Brewer, et al. have been granted a number of patents relating to the control and delivery of various defibrillation waveforms. For example, in U.S. Pat. No. 5,908,442, Brewer et al. discloses a method of delivering biphasic truncated damped sine wave shocks. Two discharge circuits that operate in succession allow delivery of biphasic wave shocks. The truncation time of the shock is determined using the Blair equivalent circuit model of defibrillation together with knowledge of distributed impedances of the chest wall, thorax, lung, and heart. This method requires that the total patient impedance be known before shock delivery.
Brewer et al., in U.S. Pat. No. 5,991,658, describes a method of continuously determining the tilt of a truncated exponential waveform based on repeated discrete measurements of the impedance or resistance of the patient. When the storage capacitors decay to the point where an optimal tilt based on defibrillation efficacy models, equals the computed tilt the waveform is then truncated.
Further, Brewer et al. in U.S. Pat. No. 5,978,706 teaches a method of continuously determining the truncation point of a damped sinusoidal waveform, similar to that described in the '442 patent, but applied to the delivery of a sinusoidal waveform. The '706 patent discloses a method of truncation that requires measurement of the patient's resistance. Specifically, a pre-calculated design rule to determine truncation time based on patient impedance that is continuously measured and discretely updated during delivery of the waveform is implemented. This method relies on a measurement of impedance prior to shock delivery, rather than a real-time impedance measurement during shock delivery.
Lerman, in U.S. Pat. Nos. 4,574,810, 4,771,781, and 5,088,489 discloses a method of delivering sinusoidal current to transthoracic defibrillation paddles/electrodes and then measuring the resultant voltage across the electrodes. This voltage is then used to determine the patient's transthoracic resistance. The resistance value is then used to scale a subsequent shock by scaling the voltage to which the capacitor is charged prior to shock delivery. The method is equivalent to a current-based process, because the peak current of the waveform becomes the controlling parameter.
Charbonnier, et al, in U.S. Pat. No. 4,328,808, proposes a method of computing transthoracic resistance given a predetermined stored energy and, by measurement of peak output current, to perform computations during delivery of a damped sine waveform. These data are used to determine delivered energy and to trigger an audible alarm if the resistance falls outside a preset boundary. In U.S. Pat No. 5,111,813, Charbonneier et al. specify an “impedance normalized delivered energy” in lieu of current.
Gliner et al., in U.S. Pat. Nos. 5,593,427, 5,601,612, 5,607,454, 5,620,470, 5,735,879, 5,749,904, 5,749,905, 5,776,166, 5,803,927, 5,836,978, 6,047,212, disclose a method for delivering a truncated exponential waveform to a patient. As the pulse is delivered, the voltage remaining on the storage capacitor is monitored. Under certain circumstances, the waveform or its first phase is truncated when the voltage decays to a certain value. However, if too little or too much time passes, the waveform may be truncated early or late.
Lopin and Avati, in U.S. Pat. Nos. 5,733,310, 5,769,872, 5,797,968, 5,800,462, 5,800,463, 5,904,706, 6,096,063, describe a method of measuring patient resistance by using a “sensing pulse” applied immediately before defibrillator discharge. This pulse is applied as a voltage and the resulting current is then measured and used to compute resistance.
In U.S. Pat. No. 5,201,865, Kuehn discloses a method of measuring lead impedance by measuring the time it takes a capacitor to discharge through a precision resistor and then comparing this time to the time required to discharge the same capacitor through the patient load.
In U.S. Pat. Nos. 5,549,643 and 5,645,573, Kroll and Smith describe a method of timing the duration of a capacitor-discharge truncated exponential waveform defibrillation shock by first waiting for the capacitor voltage to decay by a certain percent. Then it extends the waveform by a fixed duration beyond this percentage.
In a Ph.D. thesis, entitled “A Controlled-Power Arbitrary Waveform Method of Defibrillation” (March 2000, Purdue University), Havel presents a method for instantaneously controlling output power to the load without measuring output current or load resistance. This method uses a pulse width modulator control scheme that uses the voltage on the storage capacitor as a feedback parameter. Thus, output power is controlled by actively calculating the rate of decay of the energy storage capacitor.
In U.S. Pat. Nos., 5,481,238, issued to Carsten, et al., U.S. Pat. No. 5,629,842, issued to Johnson, et al., and U.S. Pat. No. 5,165,162 issued to Charles, there are descriptions of how compound inductors may be assembled in buck and boost regulators. For example, a toroidal inductor member formed from a plurality of turns of wire is described in the '842 patent, including an inductor with a segmented toroidal core with a winding wound thereon in the '162 patent.
Typically, ICDs have the capability of providing a variety of defibrillation waveforms. In the main, these waveforms have either been monophasic or biphasic waveforms applied as truncated exponential waveform pulses. Clinically, however, there is a need for an apparatus and method that would take account of changes in patient resistance. A patient's impedance changes due to any of a wide variety of causes, the defibrillation waveform pulse may provide far less energy than what the physician has programmed. Thus, there is a need for new methods to provide a consistent amount of energy in the presence of varying impedances, as is disclosed in the present invention.